98 PRACTICAL STRUCTURAL DESIGN 



should be as far from the neutral axis as possible, which indicates 

 the bottom of the girder as a proper location. When a seating is 

 cut into the bottom of a joist it is apt to cause the joist to split, 

 and if season checks open at this point the joist will be greatly 

 weakened. Hah the depth of a joist may be cut out at the very 

 end without weakening it for carrying purposes, for the bending 

 moment at the end is zero, but if a joist splits for any considerable 

 distance from the end it is greatly weakened. The principal 

 objection, therefore, to the nailed bearing strip is the danger of 

 splitting the joists at the upper edge of the seat. If the joists rest 

 without cutting on the bearing strip and the strip is properly de- 

 signed, it cannot be condemned. There will, however, in this case, 

 be a projection of the girder below the ceiling equal to the width 

 of the strip. 



6. Design a laminated floor to carry a total load of 48 Ibs. per 

 square foot on a span of 16 ft. Use white pine with a fiber stress 

 of 800 Ibs. per square foot. Ignore deflection. 



A laminated floor is a solid floor consisting of 2-in. planks spiked 

 side by side. The width to use in designing is 12 ins., the load 

 being in pounds per square foot. 



M = 1.5 X 48 X 16 2 = 18,432 in. Ibs. 

 -i-6 = 133. 



J 18,432 

 d = Y ' ^ = 3.4 in. 



133 x 12 



Use 2 ins. X 4 ins., which will give an actual depth of 3| ins. 

 If the deflection is not to exceed ^-g- of the span, the deflection 



Ml -L 1" X 1^ n rrti 



will be = 0.534 m. 

 oou 



TO -I ,"1 



Actual deflection = -T^T = -^ = 1.535 ins. 



4on 4o X 



The formulas previously given for deflection, page 80, are 

 based on the fiber stress and to avoid several trial computations 

 to ascertain the depth with the reduced stress use the rule that : 



The deflection in beams varies as the cube of the length in feet 

 divided by the breadth in inches multiplied by the cube of the depth 

 in inches. 



Expressed as a formula it appears : 



Depth varies as r- 



